The present invention relates generally to gamma-ray scintillation involved in well drilling or well logging operations and, more particularly, to stabilizing the gain of a gamma-ray scintillation detector involved in well drilling or well logging operations.
Spectroscopic analysis of data from nuclear well logging operations may provide quantitative or qualitative information related to a geological formation surrounding a borehole. By measuring spectra of nuclear radiation (e.g., gamma-rays) emitted by the formation and comparing the measured spectra to those of known chemical components, a general composition of the formation may be discerned. The composition of the formation may imply certain petrophysical characteristics associated with the formation, such as porosity, matrix lithology, and water salinity. Alternatively, gamma radiation or x-rays emitted by a source mounted in the logging tool can be measured by one or more detectors in the logging tool after the radiation has been scattered in the formation in such a way that it can be detected by the detector. The amount of radiation scattered back from the formation or borehole and its energy spectrum can give quantitative information on the electron density and photoelectric factor of the formation.
Radiation emitted or scattered by the formation may be measured with a scintillation detector. When radiation from the formation strikes the scintillation detector, the detector may produce an electrical signal corresponding to the energy deposited by the radiation in the scintillator. Because the scintillation detector may not operate at a predictable constant gain, techniques for stabilizing the gain have been developed. Such techniques may generally involve placing a radioactive source with a well-defined gamma-ray energy near the detector while x-ray and gamma-ray radiation emitted by or returning from the formation or the borehole is being detected. The gamma-ray peak from the stabilization source in the detected spectrum may thereafter be used as a reference for determining and stabilizing the gain of the system. Reliance on a radioactive source placed near the detector may introduce a number of disadvantages, however, as using radioactive sources may imply burdensome regulations, the sources may have limited useful lives (e.g., 1 to 15 years), the strength of the sources may need monitoring, and the encapsulation of the sources may not be guaranteed beyond a certain predefined period after the date of manufacture.